In recent years, for realization of both lower cost and higher efficiency of a solar cell, a thin film solar cell, which is made using a small amount of raw materials, has attracted attention and is under intense development. Currently, in addition to a conventional amorphous thin film solar cell, a crystalline silicon thin film solar cell has also been developed, and a stacked solar cell, formed by stacking these cells and called a hybrid solar cell, has also been put into practical use. “Crystalline silicon” or “microcrystalline silicon” is a mixed crystal system of crystal silicon and amorphous silicon, a material whose crystal volume fraction changes depending upon film formation conditions.
The thin film solar cell normally has a photoelectric conversion unit in which a p-type layer (p-type semiconductor layer), an i-type layer (i-type semiconductor layer) and an n-type layer (n-type semiconductor layer) are stacked. As for such a pin-type or nip-type photoelectric conversion unit, regardless of whether the p-type layer or the n-type layer included therein is amorphous or microcrystalline, a photoelectric conversion unit whose i-type layer, taking up a principal part thereof, is amorphous, is called an amorphous silicon photoelectric conversion unit, and a photoelectric conversion unit whose i-type layer is microcrystalline silicon made of mixed crystal of crystal silicon and amorphous silicon is called a crystalline silicon photoelectric conversion unit.
The i-type layer, which is substantially an intrinsic semiconductor layer, occupies a principal part of thickness of the photoelectric conversion unit, and photoelectric conversion mainly occurs in the i-type layer. Therefore, the i-type layer is usually referred to as an i-type photoelectric conversion layer or simply referred to as a photoelectric conversion layer. The photoelectric conversion layer is not limited to the intrinsic semiconductor layer, and it may be a slightly doped p-type or n-type layer as long as loss of light absorbed by impurities (dopant) is not problematic. While the photoelectric conversion layer preferably has a larger thickness for its better light absorption, an excessive increase of the thickness inevitably increases cost and time for depositing the layer.
One of the big challenges in mass production of the thin film solar cell is high-speed film formation of the crystalline i-type silicon photoelectric conversion layer of the crystalline silicon photoelectric conversion unit performed by a CVD method with in-plane uniformity. That is, since crystalline silicon has a small absorption coefficient as compared with amorphous silicon and the crystalline i-type silicon photoelectric conversion layer of the crystalline silicon photoelectric conversion unit is required to have a thickness on the order of ten times as large as the amorphous i-type silicon photoelectric conversion layer of the amorphous silicon photoelectric conversion unit, the film formation of the crystalline i-type silicon photoelectric conversion layer controls a speed of manufacturing of the solar cell including the crystalline silicon photoelectric conversion unit.
On the other hand, although discharge is performed on conditions of high pressure, high hydrogen dilution and high output with an interelectrode distance being in a narrowed state for the high-speed film formation of crystalline silicon (e.g., Patent Document 1, Non-patent Document 1 and Non-patent Document 2), grain boundaries of microcrystalline silicon are prone to be generated and defects are prone to be concentrated on the crystal grain boundaries under the high-speed film formation conditions, thereby causing a problem that a fill factor and an open circuit voltage of the solar cell are prone to decrease. For reduction in density of the defects attributed to the crystal boundaries of the crystalline i-type silicon photoelectric conversion layer, a dilution ratio of hydrogen may be decreased. However, when the dilution ratio of hydrogen is decreased, the crystal volume fraction tends to decrease and become non-uniform, leading to a decrease in short-circuit current of the solar cell. Further, the crystalline silicon photoelectric conversion unit with a low crystal volume fraction also has a problem of being sensitive to photodegradation.
Meanwhile, although the film thickness of the photoelectric conversion layer is preferably large in order to increase light absorption by the i-type layer so as to enhance the photoelectric conversion efficiency of the solar cell, when the film thickness of the i-type layer is made larger than necessary, the cost and the time for the film formation increases. From such a viewpoint, manufacturing conditions for the crystalline i-type silicon photoelectric conversion layer has hitherto been selected so as to balance a film formation speed and a film quality thereof. That is, for formation of a crystalline i-type silicon photoelectric conversion layer with high film quality, the film formation speed needs to be lowered, and when the film formation speed is prioritized, the film quality deteriorates, thus requiring formation of the crystalline i-type silicon photoelectric conversion layer with a large film thickness. Therefore, an attempt has been made to adjust the film formation conditions for the crystalline i-type silicon photoelectric conversion layer and the film thickness thereof in the trade-off relation between the film formation speed and the film quality, so as to optimize the characteristics and the mass productivity of the solar cell.
On the other hand, it has been proposed that the n-type amorphous silicon layer be formed on the n-type crystalline silicon layer and the crystalline silicon photoelectric conversion layer be formed thereon, thus reducing the crystal grain boundaries and defects in the crystalline silicon photoelectric conversion layer so as to improve photoelectric conversion characteristics (e.g., Patent Document 2). In Patent Document 2, an n-type amorphous silicon layer containing a P element, which functions as a crystallization promoter, acts as an underlayer of the crystalline silicon photoelectric conversion layer. Hence a crystalline photoelectric conversion layer of good quality, in which crystal nucleus generation in the early stage of growth is suppressed, can be obtained and the photoelectric conversion characteristics are improved.